Gas generant formulations for automotive airbags contain as a minimum, a fuel and an oxidizer. Additionally it may contain other ingredients to modify the nature of the slag produced in the combustion process, to increase the burning rate, to cool the composition, or to function as a processing aid. Such formulations are commonly formed into pellets for insertion into an inflator device by rotary pressing equipment or other pressing equipment using a system of dies and punches as described for example in U.S. Pat. Nos. 4,561,675 and 4,547,342, the teachings of each of which are incorporated herein by reference. Gas generants containing significant levels of metal oxides present as hard particles are pressed into pellets with great difficulty as manifest by the high release load required to remove the pellets from the dies. This in turn is manifest in a high rate of wear on the dies and punches. It is common practice to include processing aids such as water, graphite powder, molybdenum disulfide, boron nitride, or salts of fatty acids into the formulations to reduce the force required to remove the pellets from the dies, and hence results in a reduction in tool wear which also reduces the cost of producing the gas generant. Many compositions cannot be mass produced into pellets without the use of a processing aid and thus processing aids are a very important part of the gas generant formulation.
It is recognized by those skilled in the art that processing aids themselves either become fuels, oxidizers, or inert ingredients within any gas generant formulation and contribute to the overall properties of the composition such as burning rate, mechanical strength, gas toxicity, and ability to form easily filterable slag. In general it is most desirable to use a processing aid at the lowest level possible. Blending the processing aid to a previously prepared gas generant powder of prilled composition rather than incorporating the processing aid into the bulk composition also greatly increases its effectiveness.
Salts of fatty acids (for example, calcium or magnesium stearate) used in formulations containing transition metal oxides have proven effective in decreasing mold release forces when used at levels in the range of 1% by wt. The same formulation by way of comparison requires from 1.5 to 3.0% by weight of molybdenum disulfide to produce a comparable effectiveness in decreasing mold release forces. The fatty acid salts, however, reduced the burning rate of the formulation to undesirable levels relative to formulations with molybdenum disulfide. For this particular composition it would be most desirable to have the effectiveness of the calcium stearate without the consequent loss of burning rate.